“What we don’t know is what is the mechanism behind it, why is air pollution leading to increased risk of heart attack and stroke?” said Dr Nay Aung, a cardiologist at Queen Mary University of London and first author of the research.

The latest study helps to unpick the conundrum. Writing in the journal Circulation, Aung and colleagues report that they found exposure to nitrogen dioxide and fine particulate matter, known as PM2.5 and PM10 particles, is linked to an increase in the size of two of the chambers of the heart, the left and right ventricle. PM particles are commonly emitted by motor vehicles, among other sources.

The authors add that similar changes can affect the performance of the heart and are often seen before heart failure takes hold.

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The team used data from almost 4,000 volunteers who were part of a wider research effort known as the UK Biobank. These participants were aged between 40 and 69 years old, had been at the same address for the whole study, and were free from cardiovascular disease at the outset. Crucially, their data included cardiac MRI scans, which offer detailed images of the structure and function of the heart.

The study also involved estimates of the outdoor concentrations of different pollutants at participants’ home addresses at about five years prior to the scan. After controlling for factors including age, sex, income and smoking history, the team found that higher exposure to PM2.5 particles, PM10 particles and nitrogen dioxide were each linked to a greater volume of both the right and left ventricles after they had filled with blood.

Aung said the size of the effect identified was small, but important. “This effect size is comparable to other well known cardiac risk factors such as hypertension,” he said, noting that as blood pressure rises the heart size increases. “Although the increase in heart chamber size is small in this study, it is an early warning sign, which may explain the increased risk of heart failure in individuals exposed to higher level of pollution.”

“We know that people with heart failure or people who are developing heart failure, their heart will undergo changes, and one of the changes is they become larger,” said Aung. “When they become larger, that means that the heart is under stress, so the only way to accommodate this increased pressure and volume is to become larger. If you don’t treat or reverse that change, in the long run the heart may fail.”

Aung said the study found that an increase in exposure to PM2.5 of 1µg/m3 was linked to an increase in the size of each ventricle of just under 1%. He stressed that the findings were of particular concern because most of the participants lived in areas with relatively low exposure to air pollution.

On average, the participants were exposed to average PM2.5 concentrations of 8-12µg per cubic metre, close to the WHO recommended limit of 10µg/m3, but well within the UK guidelines of 25µg/m3. Research last year found that in some polluted areas such as central London, average levels of PM2.5 were above 18µg/ m3, with even higher levels seen on bad pollution days.

Previous work has also shown that mice exposed to high concentrations of PM2.5 develop larger left ventricles, the authors note.

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Chris Gale, professor of cardiovascular medicine at University of Leeds who was not involved in the study, said that since people travel for work and other purposes, the home addresses of the participants might not represent the main location where they encountered air pollution. What’s more, he said the study looks at a snapshot in time, rather than recruiting and following individuals over years.

Nonetheless, he said the study was important: “[It] offers a possible mechanistic clinical pathway between the detrimental effects of air pollution and cardiovascular disease,” he said.

Kevin McConway, emeritus professor of Applied Statistics at the Open University, welcomed the study. “I think the study provides pretty convincing evidence of a correlation between levels of two air pollutants – fine particulates in the air and nitrogen dioxide – and measurable changes in the heart,” he said.

But, he added, the study only shows a link, rather than showing that it is air pollution that is driving the heart changes. “In this study the researchers did adjust their results carefully to allow for possible effects of many factors to do with lifestyle,” he said. “Such statistical adjustments can never be perfect, though, so some doubt must remain about whether the heart changes are actually caused by the pollution.”

Aung said that while the government’s recent consultation on clear air is a step in the right direction, avoiding air pollution can be difficult and individuals should also improve their health through other measures including tackling issues such as obesity, high cholesterol or high blood pressure.

Katie Nield of environmental legal group ClientEarth said: “This study is particularly worrying as it shows the serious health effects of air pollution at levels well below the current legal limits. We need a new Clean Air Act with stricter, binding air quality standards that reflect the latest science and help protect people from the serious damage that air pollution does to their health.”